Gas-tight diaphragms for electrochemical cells

ABSTRACT

DESCRIBED IS A GAS-TIGHT DIAPHRAGM MATERIAL OF HIGH MECHANICAL STABILITY COMPRISED OF FIBROUS MATERIAL AND A SYNTHETIC BINDER. THE MATERIAL IS CHARACTERIZED BY THE FACT THAT THE BINDER THEREFOR CONSISTS OF A COPOLYMER PRODUCED BY THE SAPONIFICATION OF THE NITRILE GROUPS OF A BUTADIENE/ STYRENE/ACRYLONITRILE PRECURSOR AND THAT THE CONTENT OF THE BINDER IS FROM 6 TO 15% BY WEIGHT AND PARTICULARLY 8 TO 12% RELATIVE TO THE WEIGHT OF THE FIBER MATERIAL.

United States Patent Ofice Patented June 8, 1971 US. Cl. 162-155 5 Claims ABSTRACT OF THE DISCLOSURE Described is a gas-tight diaphragm material of high mechanical stability comprised of fibrous material and a synthetic binder. The material is characterized by the fact that the binder therefor consists of a copolymer produced by the saponification of the nitrile groups of a butadiene/ styrene/acrylonitrile precursor and that the content of the binder is from 6 to by weight and particularly 8 to 12% relative to the weight of the fiber material.

It is known to use asbestos diaphragms in electrochemical cells, particularly in fuel cells. For example in fuel cells described in the French patent of addition No. 87,850, asbestos diaphragms are inserted between a supporting structure containing the electrolyte and the electrodes which border both sides, in order to reduce the danger of penetration of the reaction gas into the electrolyte.

It is also known that conventional asbestos diaphragms can only under specific operational conditions meet the demands placed upon fuel elements, The asbestos fibers or the asbestos papers swell strongly under the influence of an alkaline electrolyte, forming a fleece of unbound fibers which may cause considerable disturbance during the operation of the fuel element. Bound, commercially available, asbestos papers possess a low chemical stability of the binding means with respect to the electrolytes. Re-

action or degradation products from reactions between the binder and the electrolyte may lead to other significant disturbances with respect to the electrolyte.

It has been proposed, therefore, to produce gas-tight or hermetic diaphragms for fuel element from highly pure asbestos fibers whose binder is comprised of a synthetic material, stable to the electrolytes and used in amounts of from 0.5 to 6% by weight, and particularly from 1.5 to 3% by weight, relative to the weight of the asbestos fibers. According to this proposal, aqueous synthetic latices are used for the production of diaphragms drawn on asbestos fibers dispersed in water. Suitable synthetic latices are those which are not attacked by the aqueous electrolyte and which show good stability with respect to air or oxygen.

A number of synthetic dispersions have become known which possess the required chemical stability with respect to electrolytes. Adequate chemical stability is offered by dispersions of polychloroprene, polyethylene, polytrifiuorochloroethylene, polytetrafluoroethylene, polystyrene, polymethacrylate and butadiene/styrene copolymer. The disadvantage of these synthetic materials as a binder is that they are hydrophobic and therefore can be used only in amounts up to 6% by weight of the asbestos fiber. As a result, the mechanical stability of the diaphragms is unsatisfactory in many instances. By increasing the amount of the binder added to above 6% by weight, it is possible to obtain diaphragms of high stability, however, simultaneously worsening the electric conductivity and reducing the gas tightness.

We have now found that the properties of gas-tight diaphragms for electromechanical cells, particularly for fuel elements, can be considerably improved by using a binder for the fiber material, which is comprised of a copolymer constituting hydratable groups, particularly carboxyl, hydroxy, sulfo or amino groups, with the amount of the binder used constituting 6 to 15 by weight, and particularly 8 to 12% by weight, of the weight of the fiber material.

The production of the asbestos diaphragms is effected in a known manner by drawing the polymer on a fiber material from aqueous polymer dispersions. The dispersions used therefor may be those whose polymers already contain the hydratable groups, required by the present invention, as well as those polymers wherein hydratable groups are formed through a chemical aftertreatment of the finished diaphragm.

Suitable binders are dispersions of copolymers based on styrene/styrene sulfonic acid, butadiene/styrene/styrene sulfonic acid, styrene/aminostyrene, butadiene/styrene/vinylpyridine, ethylene/vinylacetate and chlorosulfonated or carboxylated polyethylene. To build up the hermetic diaphragms it is preferable to employ dispersions of copolymers of butadiene/styrene/acrylic acid, methacrylic acid/methacrylic acid ester, methacrylic acid/ methacrylic acid aminoalkylester, methacrylic acid ester/ methacrylic acid hydroxyalkylester, for example methacrylic acid octylester/methacrylic acid Z-hydroxypropylester and methacrylic acid octylester/methacrylic acid hydroxyethylester. As esters for methacrylic acids one may use esters of aliphatic alcohols with 1 to 10 C-atoms, as well as alicyclic alcohols.

As previously mentioned, it is also possible to use as binders for the diaphragms of the present invention, such polymers whose hydratable groups are freed only in the finished diaphragms through a chemical aftertreament. Thus, ester or nitrile groups of polymers may be subsequently saponified. Particularly preferred for this purpose are the butadiene/styrene/acrylonitrile copolymers. Preferably, one will proceed in such a manner that the finished diaphragms will be brought into contact, at room or higher temperature, with the electrolytes to be used later, for example 2 N or 6 N KOH. Naturally, the freeing of the hydratable groups may be effected independently of the electrolyte.

The polymers employed in accordance with the present invention contain hydrophilic as well as hydrophobic groups. The installation of the hydrophilic groups into the binder results in the increasibility of the mechanical stability of the diaphragm by increasing the share of the binder, without at the same time impairing their wettability and thereby their gas-tightness, as well as electric conductivity. The diaphragms of the present invention are gas-tight up to above 2 atm. g. (atmospheres gauge) and operate completely undisturbed in fuel cells at approximately 70 C., in 6 N KOH, even for very long periods. In diaphragms of 0.25 to 0.6 mm. thickness, the electrical resistance is below 0259 -cm.

The mechanical stability, especially the wet break strength as well as the chemical resistance of the diaphragms may be further increased by cross-linking the copolymers employed. Thus, for example, butadiene containing copolymers may be polymerized peroxidically. The peroxides are added, for this purpose, to the binder prior to their application onto the fiber material, whereby the desired polymerization is brought about at the finished diaphragm through a thermal aftertreatment. Due to the relatively low binder content, this does not significantly affect the swellability of the diaphragms.

According to a further embodiment of the invention the following may be used to advantage in lieu of the asbestos fibers as a carrier material: fibers, fleece, felts, tissues or paper of carbon, low alkali glass or fibers of synthetic material, for example polypropylene fibers.

The object of the present invention Will be disclosed in still greater details by means of the following examples.

EXAMPLE 1 For a diaphragm of 300 cm. (36 mgr/cm. weight/ area) and approximately 0.5 mm. thickness, 12 g. dry, clean asbestos fibers are first dispersed to a slurry in 750 ml. Water. Subsequently, to this dispersion are added, by being dripped in under strong agitation, 2.4 g. of an aqueous dispersion, 40% by weight of dry substance, of a butadiene/styrene/acrylic acid copolymer (acrylic acid content -8% in copolymers). To effect complete precipitation onto the fiber, the mixture is admixed also with 25 ml. of cold saturated NH Al(SO solution.

The mixture may be immediately processed into a diaphragm on a sheet former or a similar device. If dispersions are used with self-polymerizing additions, a thermal aftertreatment is necessary, depending on the addition, up to 140 C. for 20 to 30 minutes.

EXAMPLE 2 To the fiber slurry indicated in Example 1, one adds, under strong agitation, 3.1 g. of an aqueous dispersion (30% by weight of dry substance) of a butadiene/styrene/acrylonitrile copolymer to content of acrylonitrile in the copolymer). For complete precipitation, ml. of a cold saturated NH Al(SO solution are also added. For saponification of most of the free nitrile groups of the binders to carboxyl groups (hydrophilation), the finished diaphragm is processed with 2 N KOH, at room temperature for minutes. Diaphragms thus produced are very well wetted by aqueous electrolytes and have a gas tightness up to above 2 atmospheres gauge.

EXAMPLE 3 A felt of carbon fibers (thickness of felt 1 mm., area weight 18 mg./cm. is saturated with a dispersion (10% by weight of solid substance) of butadiene/styrene/ acrylic acid copolymer.

Subsequently, the carbon felt is placed in a coagulating bath comprised of a cold saturated NH.,A1(SO.,) solution. After a single washing with distilled water it is pressed between two heatable, planar parallel plates, at 130 C. and 5-10 kp./cm. pressure, for 20 minutes. The diaphragm thus obtained has a thickness of 0.5 to 0.7 mm.

When using a binder containing nitrile groups, in accordance with Example 2, the diaphragm must be aftertreated with 2 N KOH,'whereby the saponification of the nitrile groups takes place.

EXAMPLE 4 To produce a diaphragm, 2,6 g. of an aqueous dispersion by weight of dry substance) of a butadiene/ styrene/styrene sulfonic acid copolymer (5% styrene sulfonic acid content in copolymer) are added to the fiber slurry which is produced from 12 g. of highly pure asbestos fibers in 750 ml. water. ml. 2 N H 80 are additionally admixed for a complete precipitation of the binder onto the fiber. Further processing is carried out as in Example 1.

EXAMPLE 5 To a slurry dispersion of 12 g. blue asbestos fibers in 750 ml. water, 2.9 g. of an aqueous dispersion (30% by weight of dry substance) of a methylrnethacrylate/ octylmethacrylate-dimethylaminoethylmethacrylate copolymer are added, under strong agitation. After a completed precipitation and further treatment on the sheet former, diaphragms are obtained, which are particularly well suited for electrochemical cells with acid electrolytes.

EXAMPLE 6 3.1 g. of an aqueous dispersion (2 8% by weight of dry substance) of a methylmethacrylate/dimethylaminoethy1- methacrylate copolymer are dripped into the fiber slurry of Example 5, under strong agitation. The production of these diaphragms, which are also particularly suitable for acid electrolytes, is effected analogously to Example 1.

EXAMPLE 7 2.95 g. of an aqueous dispersion (30% by weight of dry substance) of a methylmethacrylate/octylmethacrylate hydroxyethylmethacrylate copolymer are added by dripping to the fiber slurry described in Examples 1 or 5, under strong agitation. After standing for 2 hours, the mixtures are processed into diaphragms on a sheet former or another suitable device and may be employed, according to the type of asbestos fiber used, either in acid or in alkaline electrolytes.

EXAMPLE 8 3.2 g. of an aqueous dispersion (26% by weight of dry substance) of a methylmethacrylateloctylmethacrylate/2-hydroxypropylene methacrylate copolymer are added by dripping into a fiber slurry in accordance with Examples 1 and 5, under strong agitation. After standing for 3 hours, these mixtures also produce diaphragms, on the sheet former, which will be gas-tight up to above 2 atmospheres gauge.

We claim:

1. In a gas-tight diaphragm material of high mechanical stability comprised of fibrous material selected from asbestos fibers and carbon fibers, and a synthetic hinder, the improvement which comprises using a binder consisting of a copolymer which contains hydrated groups, said binder being a butadiene/styrene/acrylic acid copolymer produced by saponification of the nitrile groups of a butadiene/styrene/acrylonitrile precursor, said saponification being in the presence of said fibers, said binder being in an amount of from 6 to 15% by weight of the fibrous material.

2. The material of claim 1, wherein the binder is 8 to 12% by weight.

3. The membrane material of claim 1 wherein the binder consists of a cross-linkable butadiene/styrene/ acrylic acid copolymer.

4. The membrane material of claim 1, wherein the fibrous material forms an interfelted web.

5. The method of forming a membrane material for a gas-tight diaphragm, which comprises forming a slurry of a material selected from the group consisting of asbestos and carbon, which comprises adding from 6 to 15% by weight of a butadiene/styrene/acrylonitrile copolymer, containing hydratable groups, based upon the solids content to a dispersion of fibrous material selected from asbestos, and carbon fibers to form said slurry thereafter drawing the water from said sluury to form the membrane material and saponifying the nitrile group in the said copolymer.

References Cited UNITED STATES PATENTS 2,789,903 4/1957 Lukman et al. 162-468 2,901,390 8/1959 Conklin et al. 162-152 3,014,835 12/1961 Feigley et al. 162156X 3,057,794 10/1962 Carlin 204-296X 3,066,066 11/1962 Keim et al 162-155X 3,093,609 6/ 1963 Feigley et al. 162155X 3,144,379 8/1964 Gelbert 162-155 3,153,610 10/1964 Heiser et al 162-155 3,269,889 8/1966 Hutchins 162-155 3,275,575 9/1966 Fogle 204-296X S. LEON BASHORE, Primary Examiner F. FREI, Assistant Examiner U.S. Cl. X.R. 

